fix(frontend): allow /datamart-builder/api as valid backend base URL

The embedded mode sets VITE_BACKEND_URL=/datamart-builder/api (the nginx
proxy path), but the runtime-config policy only allowed /api. This caused
resolveBackendUrl() to throw at app startup, resulting in a blank SPA.

Add /datamart-builder/api and /datamart-builder/api/ to relativeBases and
test cases.

frontend/src/api/backend-url-cases.json

@@ -3,6 +3,8 @@
   { "value": "/", "valid": true },
   { "value": "/api", "valid": true },
   { "value": "/api/", "valid": true },
+  { "value": "/datamart-builder/api", "valid": true },
+  { "value": "/datamart-builder/api/", "valid": true },
   { "value": "http://localhost:8787", "valid": true },
   { "value": "https://api.example.test/v1", "valid": true },
   { "value": "https://api.example.test/base/path/", "valid": true },

frontend/src/api/backend-url-policy.json

@@ -1,5 +1,5 @@
 {
-  "relativeBases": ["", "/", "/api", "/api/"],
+  "relativeBases": ["", "/", "/api", "/api/", "/datamart-builder/api", "/datamart-builder/api/"],
   "absolutePattern": "^https?://(?:\\[[0-9A-Fa-f:.]+\\]|[A-Za-z0-9](?:[A-Za-z0-9.-]*[A-Za-z0-9])?)(?::[0-9]+)?(?:/[^\\s?#]*)?/?$",
   "maxPort": 65535,
   "queryAllowed": false,